Estimating ship velocity through the water and over the ground

Abstract

A method and system are provided to estimate a ship's velocity. Two ship sonar returns from two different directions are selected. Each return includes volume reverberation data and bottom reverberation data. Doppler shift for each of the two returns based on the volume reverberation data is determined as a basis for estimating ship velocity relative to the water. Doppler shift for each of the two returns based on the bottom reverberation data is determined as a basis for estimating ship velocity relative to the ground.

What is claimed is:  
     
       1. A method of estimating a ship's velocity relative to the water and relative to the ground, comprising the steps of: 
       providing a sonar system onboard a ship;  
       operating said sonar system to generate sonar returns;  
       selecting two returns from said sonar returns, said two returns originating from two different directions separated by an angle, each of said two returns including volume reverberation data and bottom reverberation data;  
       selecting said volume reverberation data and said bottom reverberation data from each of said two returns;  
       determining a Doppler shift in frequency for each of said two returns based on said volume reverberation data as a basis for estimating ship velocity relative to the water; and  
       determining a Doppler shift in frequency for each of said two returns based on said bottom reverberation data as a basis for estimating ship velocity relative to the ground.  
     
     
       2. A method according to claim  1  wherein said angle is 90°. 
     
     
       3. A method according to claim  1  wherein one of said two different directions is the heading of the ship. 
     
     
       4. A method according to claim  3  wherein said angle is 90°. 
     
     
       5. A method according to claim  1  wherein each said step of determining comprises the steps of: 
       adding said Doppler shift in frequency for each of said two returns to form a sum;  
       taking a square root of said sum; and  
       multiplying said square root by (c/2f 0 ) where c is equal to the speed of sound in water and f 0  is a transmitting frequency of said sonar system.  
     
     
       6. A method of estimating a ship's velocity relative to the water and relative to the ground, comprising the steps of: 
       providing a sonar system onboard a ship;  
       operating said sonar system at a transmitting frequency of f 0  to generate sonar returns as a function of time, each of said sonar returns having a volume reverberation spectral component and a bottom reverberation spectral component;  
       selecting a first return and a second return from said sonar returns, said first return aligned along the heading of said ship and said second return angularly spaced from said first return;  
       determining a first Doppler frequency shift Δf 1  using said volume reverberation spectral component of said first return;  
       determining a second Doppler frequency Δf 2  using said volume reverberation spectral component of said second return;  
       determining a third Doppler frequency shift Δf 3  using said bottom reverberation spectral component of said first return; and  
       determining a fourth Doppler frequency shift Δf 4  using said bottom reverberation spectral component of said second return, wherein an estimate of velocity V W  of said ship relative to the water is          V   W     =     c            (       Δ                   f   1   2       +     Δ                   f   2   2         )         2        f   0                           
       and wherein an estimate of velocity V G  of said ship relative to the ground is          V   G     =     c              (       Δ                   f   3   2       +     Δ                   f   4   2         )         2        f   0         .                       
     
     
       7. A method according to claim  6  wherein said second return is angularly spaced from said first return by 90°. 
     
     
       8. A method according to claim  6  wherein, prior to said steps of determining, said method further comprises the steps of: 
       digitizing said first return and said second return to form a first time series and a second time series, respectively;  
       selecting portions of said first time series and said second time series containing data used in generating each said volume reverberation spectral component and each said bottom reverberation spectral component;  
       statistically smoothing each of said portions; and  
       performing a spectral analysis on each of said portions so smoothed to generate said volume reverberation spectral component and said bottom reverberation spectral component associated with each of said first return and said second return.  
     
     
       9. A system for estimating a ship's velocity relative to the water and relative to the ground, comprising: 
       a sonar system mounted onboard a ship for generating sonar returns over a range of angles relative to said ship;  
       a beam selector coupled to said sonar system for selecting two returns from said sonar returns, said two returns originating from two different directions separated by an angle, each of said two returns including volume reverberation data and bottom reverberation data; and  
       a processor coupled to said beam selector for selecting space said volume reverberation data and said bottom reverberation data from each of said two returns and for determining a Doppler shift in frequency for each of said two returns based on said volume reverberation data as a basis for estimating ship velocity relative to the water, and determining a Doppler shift in frequency for each of said two returns based on said bottom reverberation data as a basis for estimating ship velocity relative to the ground.  
     
     
       10. A system as in claim  9  wherein said beam selector is a multichannel switch. 
     
     
       11. A system as in claim  9  further comprising an analog-to-digital converter coupled between said beam selector and said processor for digitizing each of said two returns provided to said processor.